Damascene processing is a method for forming metal lines on integrated circuits. It is often used because it requires fewer processing steps than other methods and offers a high yield. Through-Silicon-Vias (TSVs) are sometimes used in conjunction with Damascene processing to create three-dimensional (3D) packages and 3D integrated circuits by providing interconnection of vertically aligned electronic devices through internal wiring. Such 3D packages and 3D integrated circuits may significantly reduce the complexity and overall dimensions of a multi-chip electronic circuit. Conductive routes on the surface of an integrated circuit formed during Damascene processing or in TSVs are commonly filled with copper.
A TSV is a vertical electrical connection passing completely through a silicon wafer or die. A typical TSV process involves forming TSV holes and depositing a conformal diffusion barrier and conductive seed layers, followed by filling of the TSV holes with a metal. Copper is typically used as the conductive metal in TSV fill as it supports the high current densities experienced at complex integration, such as for 3D packages and 3D integrated circuits. Copper also supports high device speeds. Furthermore, copper has good thermal conductivity and is available in a highly pure state.
TSV holes typically have high aspect ratios which makes void-free deposition of copper into such structures a challenging task. Chemical vapor deposition (CVD) of copper requires complex and expensive precursors, while physical vapor deposition (PVD) often results in voids and limited step coverage. Electroplating is a more common method of depositing copper into TSV structures; however, electroplating also presents a set of challenges because of the TSV's large size and high aspect ratio.
In a typical TSV electrofilling process, the substrate is negatively electrically biased and is contacted with a plating solution which generally includes copper sulfate or copper methane sulphonate as a source of copper ions, as well as sulfuric acid or methane sulfonic acid for controlling conductivity, along with chloride ions and organic additives in various functional classes, known as suppressors, accelerators and levelers. The concentration of these plating bath components typically changes over the course of processing as the components are incorporated into the plated substrate, degrade over time, etc. As such, in order to achieve consistently satisfactory fill results it is necessary to monitor the composition of the bath over time. In this way, when the concentration of a plating bath additive is found to be too low, for example, appropriate steps can be taken to increase the concentration of that additive in the bath.
Widely used conventional methods for monitoring plating baths typically utilize scanning voltammetric coulometry, electrochemical titrations, spectroscopic methods (e.g., visible, IR and UV solution analysis), and various forms of HPLC to independently attempt to evaluate the concentration of various known bath components (e.g., metal, acid, and each additive) at concentrations close to the target operating concentrations. For example, in the voltammetric coulometry method, a platinum rotating disk electrode (RDE) is used as a working electrode. A signal is generated by integrating the charge passed during the anodic stripping wave of a cyclic voltammogram. Typically, a series of similar experiments are performed where the concentration of a target species in solution is modified. The solution will generally be largely insensitive to the concentration of other (non-target) bath species.
As one example, a surface that is more accelerated will exhibit faster copper plating, and the system will pass more charge during stripping. As such, a solution having an excess concentration of suppressor and a relatively higher concentration of accelerator will tend to show larger deposition and stripping charge than a solution having relatively less accelerator. As such, this type of solution may be used to measure the concentration of accelerator in solution by comparing the electrochemical response of the solution to the responses seen in a series of solutions having known levels of accelerator. The concentrations of accelerator and suppressor are determined using standard addition methods in which the ability of the plating bath to accelerate or suppress plating is evaluated relative to standard solutions. Other methods can also be used, but they likewise do not indicate the potential of the combined species in the bath in its current state (having both known/recognized and unrecognized species present). These conventional methods are able to provide reasonably accurate determinations regarding the total amount of accelerator or suppressor in the bath (though in some cases breakdown products may interfere, leading to a false signal). However, although the conventional methods are usually fairly accurate, they are not sufficiently precise to enable detection of small perturbations in the bath chemistry (e.g., the formation of very low levels of plating bath breakdown products), and they do not indicate the presence of unrecognized, potentially process poisoning or other deleterious species. These composition perturbations, though relatively small, can lead to failure in the TSV fill process.
For example, the breakdown of a small amount of accelerator can produce products with incomplete fills. Further, the loss of certain moieties responsible for maintaining suppression over long time intervals can result in incomplete fills. The addition of trace amounts of leveler moieties can likewise result in incomplete TSV fills. Further, the presence of various unrecognized materials can lead to fill failure. Each of these problems can occur at concentration changes/levels that are not detectable by conventional methods. In other words, the TSV fill process is more sensitive to changes in the bath composition than the conventional composition monitoring methods are. Thus, the conventional metrology methods are unable to accurately predict whether a particular plating bath will produce an acceptable bottom-up fill result, and can lead to the production of sub-standard devices or even the complete loss of valuable substrates.
More robust control over the quality of the filling process within an individual wafer and over the course of plating multiple wafers on a plating tool is desired. Specifically, a method that indicates whether a particular plating bath will (or will not) meet a defined electroplating specification (e.g., produce a successful bottom-up fill), that does not rely on the specifics of any particular additive constituent, additive concentration or compositions, and that does not require individually testing for the presence of different species, is desired. The disclosed techniques meet these criteria, and in particular, can be performed without knowledge of the identity of the specific species that may be present in the solution.